NASA said on Tuesday that it was abandoning efforts to get back in touch with Spirit, one of the two rovers on Mars. Spirit, which has been stuck in a sand trap for two years, fell silent last year as winter arrived and its solar panels could no longer generate enough electricity. Engineers had hoped that the rover would revive when spring returned, but they never heard from it again.Now, as the Martian days grow shorter, Spirit's managers decided that it was not worth the time and money to continue.